Apparatus and method for radio frequency heating of hydrocarbonaceous earth formations including an impedance matching technique

Abstract

The disclosure relates to a technique for radio frequency heating of hydrocarbonaceous earth formations in which a high power radio frequency transmitter is impedance matched to a transmission line including a plurality of conductors at least partially embedded in the formation to be heated. The impedance matching may be effected by a "T" network having three variable reactances. In accordance with the teachings of the present invention, continuous variations of impedance, of the type encountered during the heating of the formation, may be matched in unambiguously defined Smith chart regions by varying two of the three reactances to minimize reflected power from the transmission line.

What is claimed is: 
     
       1. An impedance matching network having a first port connected to a high power radio frequency transmitter and a second port connected to a transmission line including conductors at least partially embedded in a hydrocarbonaceous earth formation to be heated, comprising a first and a second variable reactance means, said first and second variable reactance means comprising respectively, first and second independently variable capacitors, connected in series to form an upper leg of a T network; and a third variable reactance means, said third variable reactance means comprising a third independently variable capacitor, in shunt between the first and second variable reactance means to form the central leg of a T network, wherein the impedance presented by said transmission line during the heating of the hydrocarbonaceous formation is matched by holding one of the variable reactances at its minimum or maximum and adjusting the other two variable reactance means. 
     
     
       2. The apparatus of claim 1 wherein said first variable reactance means further comprises a first fixed inductor connected in series with the first variable capacitor in the upper leg of the T network, and wherein said second variable reactance means further comprises a second fixed inductor connected in series with the second variable capacitor. 
     
     
       3. The apparatus of claim 2 wherein the series combination of the first variable capacitor and first fixed inductor has a reactance, jX 1 , which is inductive; and wherein the series combination of the second variable capacitor and second fixed inductor has an impedance, jX 2 , which is inductive. 
     
     
       4. The apparatus of claim 2 wherein the series combination of the first variable capacitor and first fixed inductor has a reactance, jX 1 , which is variable from approximately zero to a predetermined inductive reactance; and wherein the series combination of the second variable capacitor and second fixed inductor has a reactance, jX 2 , which is variable from approximately zero to a predetermined inductive reactance. 
     
     
       5. The apparatus of claim 4 wherein the reactances of the first, second and third variable reactance means are given by the equations: 0<jX 1  <200   0<jX 2  <200   -200<jX 3  <-25 Where the values of the reactances are in ohms at the frequency of the transmitter and the output impedance of the transmitter 50 ohms.     
     
     
       6. The apparatus of claim 1 further comprising means coupled at the first port of the matching network, for detecting variations in the power reflected by said transmission line during the heating of the hydrocarbonaceous formation. 
     
     
       7. The apparatus of claim 6 further comprising means for detecting the reactances of each of the three variable reactance means when the impedance of the transmission line is matched. 
     
     
       8. The apparatus of claim 7 further comprising means for selecting two of the three variable capacitors for adjustment responsive to the detected reactances of the three variable reactance means and for controlling the adjustment of said two variable capacitors responsive to detected variations in the power reflected by said transmission line. 
     
     
       9. The apparatus of claim 7 further comprising means for determining the impedance of the transmission line when matched by the impedance matching network responsive to the indicated reactances of each of the three variable reactance means.